1. Technical field
The present invention relates to a method for mounting, on a printed circuit support, electronic microcomponents to be fixed flat and having, on their lower face, contacts which can be soldered for connecting them to the printed circuit. It also relates to a circuit obtainable using this method. It is intended to make it possible to connect a microcomponent having a heat expansion coefficient appreciably different from that of the support, even if it has large dimensions.
A particularly important, although not exclusive, application of the method consists of mounting "chip-carriers" devoid of connection lugs and whose size is very much less than that of the conventional integrated circuit chips. It is however applicable to other microcomponents whose material is poorly adapted, from the thermal expansion point of view, to the support receiving them. This is the case for example for a variety of passive circuits (resistance networks in particular) to be fixed on a ceramic substrate or on a printed circuit, having metal-coated connecting notches at the periphery thereof.
The chip carriers have a ceramic base with, on its lower face, contacts for fixing it flat on a support. The difference between the expansion coefficients of the ceramic and of the glass-epoxy or glass-polyimide laminate which forms most of the usual printed circuit supports leads to assemblies which cannot stand thermal cycles. In particular, the stresses due to the differential expansion risk destroying the contacts.
2. Prior Art
Different remedies have been proposed for overcoming the above-mentioned problem. Carriers have in particular been formed whose connecting lugs have a so-called "gull wing shape" so that they can tolerate an expansion differential between the base and support. The flexibility of these lugs is limited and there only exist some sizes of chip carriers having such lugs. Another solution for overcoming the stress phenomena of thermal origin is using a support made of a material having a heat expansion coefficient comparable to that of ceramic. In particular, supports have been used formed from quartz, co-laminate products, synthetic materials such as "kevlar". These supports are expensive and difficult to obtain in large sizes.
French Application No. FR-A-2 523 397 describes a method for solving the problem of differential heat expansion in cases where the support has soldering tabs between which the equipotential connections are formed as insulated flexible wire sections rather than as printed tracks. In this method, the component is fixed flat on the support by mechanical connecting means capable of tolerating differential expansions, formed by an intermediate element made of a material thermally compatible with the microcomponent and electric connections are formed by soldering the contacts of the microcomponent on a conducting network formed on the element and extending beyond the component. This method is of particular interest for circuits to be manufactured in small numbers, due to the number of connections to be made manually.